Lmlanphere, I'll guess at this and someone with more knowledge can correct me.
I think the effect intensifies because the air currents have formed a vortex (in this case a "ring" vortex). Once a vortex is formed it's almost self-sustaining as long as the pressure differentials that created it, are still acting upon it.
When a vortex is formed it creates a very organized airflow pattern in response to the pressure differentials acting upon it. As long as the pressure differential remains, the velocity of the air flow increases within the vortex in an attempt to equalize the pressure. Anyone familiar with a tornado can readily understand the velocities that can develop in a vortex. Higher velocities mean more air mass is being moved in the attempt to equalize the pressure. It's this ever increasing velocity that plays havoc with the AOA and increased drag on the rotor blades.
I might add that even when the pressure differential is removed (such as a wing tip vortex on a FW aircraft) the vortex will sustain it's organized airflow for a time due to its rotational momentum. Without the energy input from the pressure differential, the vortex will dissipate. However with sustained energy input from the pressure differential, it intensifies.
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Safe flying to you...
[This message has been edited by Flight Safety (edited 21 January 2001).]